I'm sure others will correct me if I'm wrong, but I think I recall reading that male ducks and male chickens have different sexual "plumbing", the ducks' being much, much larger and differently shaped. Therefore, a hen, even a full grown adult hen, could be at tremendous risk of injury, even death, should your two male ducks attempt to mate her since she isn't designed to receive a male duck.

This precludes allowing male ducks live in the same pen as female chickens. If you're intent on having a chicken, you need to rethink the living arrangement.